A capacitor is an electronic device which consists of two plates of electrically conductive material separated by an insulator. When introduced in an electrical circuit the plates are connected to a negative and a positive terminal of the current source, and therefore are called cathode and anode respectively.
An electrolytic capacitor is a capacitor in which one plate is metallic and the other plate is an electrolyte. Intervening between the two plates is a dielectric consisting of a surface oxide coating on the metal plate. Conventionally, the metal plate on which the dielectric coating is formed is referred to as the anode. The term “anode” is used herein to refer both to the metal plate itself and to a combination of the metal plate with the dielectric coating. It will be clear from the context which meaning of “anode” is intended. A transition between ionic conduction in the electrolyte and electronic conduction in an external circuit is provided by a second metal plate which is referred to herein as cathode. The anode and the cathode are referred to herein collectively as electrodes. Typically, the anode and the cathode are thin foils.
Typically also, the metal of the anode is a valve metal, i.e. a metal which, when oxidized, allows current to pass if used as cathode but opposes the flow of current when used as anode. Examples of valve metals include magnesium, thorium, cadmium, tungsten, tin, iron, silver, silicon, tantalum, titanium, aluminum, zirconium and niobium.
As is the case with capacitors generally, the capacitance of an electrolytic capacitor is proportional to the surface areas of its two plates. Conventionally, the surface areas of the foils are increased by etching. High purity valve metal foils are etched electrochemically in a chloride solution with DC, AC, or an alteration of DC and AC, or a concurring AC and DC. A disadvantage of electrochemical etching is that in the case of thin foils, it weakens the foil mechanically, making it difficult to fabricate electrolytic capacitors by high speed winding. Furthermore, the electrochemical etching results in acid waste and therefore is not environmentally friendly. Recently, vacuum deposition process has been proposed for increasing the surface areas of foil electrodes.
Kakinoki et al., in U.S. Pat. No. 4,970,626, disclose a method for producing an electrolytic capacitor comprising deposition of titanium onto an aluminum substrate in an atmosphere of argon, wherein the deposition occurs at a constant or gradually varied deposition angle. The disclosed method allows producing a coating with a rough column-like structure deposited onto the substrate, which can be used as a cathode of an electrolytic capacitor. In another embodiment, a two-stage deposition process is employed at different angles at each stage. The disadvantages of the disclosed method of increasing the surface area of a foil electrode include the additional cost of working with two metals and the decrease in stability associated with an intermetallic potential.
U.S. Pat. No. 4,309,810 (Drake) teaches vacuum deposition of a metal vapor, e.g. tantalum, at a low angle onto a foil substrate, e.g. aluminum, so as to produce a column-like structure. The deposition is carried out in presence of oxygen at a partial pressure not exceeding 10−4 Torr. In an embodiment, a small quantity of an inert gas is added to the oxygen. Drake's foil has been found to be too brittle for use in electrolytic capacitors: it breaks when it is rolled into a cylindrical roll, the standard shape of an electrolytic capacitor. A similar process for producing a column-like surface structure is disclosed in DE 4,127,743 (Neumann et al). Unlike the Drake process the deposition occurs at angles greater than 30°.
Allegret et al., in U.S. Pat. Nos. 5,431,971 and 5,482,743, disclose a process of evaporating aluminum in an oxidizing atmosphere under a pressure of 0.8 to 2.3 Pa, thereby depositing a mixture of aluminum and aluminum oxide. The deposited layer consists of grain agglomerates, forming a porous matrix of aluminum oxide containing metallic aluminum crystallites embedded randomly inside these grains. Such mixed Al/Al2O3 surfaces are more robust mechanically compared to pure aluminum surfaces; however electrolytic capacitors incorporating them are known to have relatively high resistive losses and relatively low stability over time. In addition, the presence of both aluminum and large quantities of aluminum oxide in the surface of the foil makes difficult and less effective both stabilization by subsequent conventional chemical or electrochemical treatments and structure coarsening by subsequent annealing.
U.S. Pat. No. 6,764,712 (Katsir, et al.) discloses a process for increasing the surface area of a metal substrate, comprising a reactive vacuum vapor deposition of valve metal onto a metal foil in an atmosphere mainly containing oxygen and an inert gas. The process can produce high surface area coatings with a fractal-like structure characterizing by cauliflower-like morphology, which may constitute a component of integrated electrolytic capacitors described in U.S. Pat. No. 6,865,071 (Katsir, et al.)
If the foil is intended to serve as an anode of an electrolytic capacitor, the step of increasing the surface must be followed by a step of oxidizing the surface to produce a thin oxide layer, conventionally by anodization. Patents in this art include U.S. Pat. No. 4,537,665 and U.S. Pat. No. 4,582,574 (Nguyen et al.), and U.S. Pat. No. 5,643,432 (Qiu). For cathode foils, in order to maintain capacity over time, the oxidizing step is necessary, and is also termed stabilizing passivation.
A printing page which comprises a substantially planar substrate and a porous vacuum deposited ungrained coating thereon is described and claimed in U.S. Publication No. 2006-0086273 A1 published on Apr. 27, 2006 (corresponding with the GB priority application No. 0421810.3) which has inventorship in common with the present patent application.
The entire contents of the above-mentioned US patents and published US patent application are incorporated herein by reference.